The worldwide research and development effort on microdevices (e.g., electromechanical, hydromechanical, thermomechanical, electrochemical, thermoelectrical, etc.) has increased dramatically over the past decade. Such devices have found significant use as sensors in automotive and medical applications, with estimates of the global MEMS (microelectromechanical systems) market ranging from $12–14 billion in 2000. However, a far larger untapped potential exists for the use of new micromechanical devices in a variety of advanced applications, such as in: i) medicine (e.g., targeted drug or radiation delivery, rapid clinical and genomic analyses, in vitro sensors, microtools for surgery, micropumps and microvalves, microreactors, microcomponents used in biomedical imaging, etc.), ii) transportation and energy production (e.g., new sensors and actuators for pollution control, enhanced energy utilization, and improved engine performance; microcomponents for automotive, diesel, jet, or rocket engines; microcomponents for turbines used in energy conversion or generation; microreactors, micropumps, microbearings, etc.), iii) communications and computing (e.g., micro-optical devices, microactuators, microswitches, microtransducers, etc.), iv) the production/manufacturing of food, chemicals, and materials (e.g., micro-robotics, rapid on-line microsensors, microreactors, micropumps, microdies, etc.), and a variety of other consumer products (e.g., for lighting, portable electrical devices, etc.).
Despite the recognized technological and economic significance of new microdevices, the commercial fabrication methods used to date have been largely limited to techniques developed within the microelectronics industry (e.g., micromachining of silicon by photolithography/chemical etching; reactive ion etching; x-ray lithography/electroplating, etc.). While appropriate for the manufacturing of planar electronic devices, such methods are not well suited for the rapid, low-cost mass production of three-dimensional microcomponents with complex, non-planar geometries. Furthermore, the properties of silicon (room temperature brittleness, poor creep resistance at greater than or equal to 600° C., high thermal conductivity, modest melting point, biochemical incompatibility, etc.) make silicon-based microdevices unattractive for a number of potential applications. New fabrication methods capable of yielding self-assembled, non-silicon microdevices in a massively parallel fashion are needed to allow for a much wider range of commercial applications.
A significant level of worldwide activity has been undertaken to develop genetically-engineered drugs or plants. However, relatively little work has been conducted to date to develop “Genetically-Engineered Materials” (“GEMS”). That is, research and development is lacking on the use of biological systems to mass produce microcomponents or microdevices comprised of advanced materials with very controlled, fine-scale, 3-dimensional structures in a very inexpensive, reproducible manner. The purpose of the present invention is to provide a novel approach for converting 3-dimensional, biologically-derived micro- and nano-templates into new materials with a retention of shape/dimensions and morphological features. The ultimate objective of this approach is to mass-produce micro- and nano-templates of tailored shapes through the biological reproduction of naturally-occurring or genetically-tailored organisms, and then conversion of such templates by controlled chemical reaction(s) into near net-shaped, micro- and nano-components of desired compositions. In other words, the goal is to develop a new process that utilizes a unique combination of the attractive features of biological systems (e.g., the low-cost culturing of biological organisms for the rapid mass production of templates with precise retention of naturally-occurring or genetically-tailored shapes and surface features), and/or genetic engineering (e.g., the versatile tailoring of genomes of biological organisms capable of producing templates with a wide variety of shapes, dimensions, and surface features); and net-shape reaction processing (e.g., for the conversion of mass-produced, biologically-derived templates into near net-shaped components comprised of new materials with enhanced properties for a given application).
Certain biological systems are capable of reproducibly generating complex micro- and nano-scaled assemblies with a high degree of precision. An example of a biological system with very reproducible, yet complex and fine shapes and morphologies is the diatom Bacillariophyceae. Diatoms are “microscopic (1–500 micrometers in length) single-celled algae with characteristic rigid cell walls (frustules) compose of amorphous silica” (from a paper by J. Parkinson, R. Gordon entitled “Beyond Micromachining: The Potential of Diatoms,” Trends in Biotechnology, Volume 17, Number 5, pp. 190–196, 1999) (hereby incorporated by reference). Diatoms exist in large numbers in a variety of aquatic environments and are believed to account for about 25% of the world's annual production of primary carbon. Diatoms have been classified on the basis of the shape of the silica frustule, with each species of diatom exhibiting a particular, reproducible frustule shape. Two general frustule shape categories are: 1) centric diatoms that have radially-symmetric frustules, and 2) pennate diatoms that are elongated and tend to have parallel rows of holes in the silica frustule, with the rows of holes tending to be oriented perpendicular to the elongated axis. Some diatom species also exhibit patterned arrangements of multiple frustules (e.g., helical-shaped clusters of frustules). In addition to having particular frustule shapes with sizes typically ranging from about 1 to about 500 micrometers in length, the diatoms have very complex, reproducible, fine (submicron) surface features (pores, ridges, nodules, protuberances, etc.). For example, the spacing between rows of pores in a diatom frustule typically may be only about 0.3 to 2 micrometers, depending on the diatom species. The pores in the frustule wall may also be on the order of 100 nanometers (0.1 micrometers) in diameter or smaller. Furthermore, the walls of diatom frustules are comprised of nanospheres (typically about 101–102 nm in diameter).
Parkinson and Gordon have recently discussed the attractive benefits of using diatoms as materials or microcomponents for certain applications (Trends in Biotechnology, Volume 17, Number 5, pp. 190–196 (1999). Although the reproduction rate of diatoms varies with species and environmental conditions (e.g., temperature, nutrient concentration, concentration of the silicon source, etc.), typical reproduction rates range from 1 to 8 times per day. Since asexual reproduction results in repeated doubling of the number of diatoms (2 to 4 to 8 to 16 . . . etc.), such a reproduction rate may yield large numbers of diatoms in a relatively short time (e.g., billions of diatoms within a few weeks). For example, at a reproduction rate of three times per day, the number of similarly-shaped frustules generated in ten days would exceed 1 billion (30 doublings=230=1,073,741,824). The combination of asexual and sexual reproduction results in diatom frustules of similar shape with a relatively narrow distribution of sizes. The variation in surface features (e.g., pore sizes, spacing between pores) may be even smaller. Once the relevant biochemistry and genetic code is understood for the shapes of diatom frustules and, in particular, how such a code may be altered to produce desired (tailored) frustule shapes and features, then shape-tailored diatom frustule templates could be produced at low cost, in large quantities, and in very reproducible shapes and very fine geometries. Control of features at submicron and nanometer dimensions would be possible while achieving economy of scale. Such a high rate of reproduction of self-assembling, complex three-dimensional shapes is an inherent biological characteristic that is highly attractive from a manufacturing perspective.
Given the wide natural variations observed in the shapes and surface features of diatom frustules, genetic engineering may be used to produce a wide variety of tailored frustule shapes (e.g., microtubes, microgears, microwheels, micropins, microsprings, microrotors, microballs, microsyringes, microcapsules, etc.). In other words, the fact that there are an estimated 100,000 species of diatoms, with each species possessing a unique frustule shape, allows new, non-deadly genetic modifications may be made to existing diatoms, so as to produce new living diatoms that possess frustules with new shapes that are appropriate for desired applications. With the understanding of the biochemistry and genetic factors responsible for the diatom shape, genetic engineering may be used to produce diatom frustules with an even wider variety of complex shapes, dimensions, and surface features than are currently available in nature. Biological reproduction (which may involve cloning) of a genetically-tailored diatom cell may then be used to generate large numbers of similar shape-tailored frustules.
Biochemical mechanisms responsible for the formation of diatom frustules are becoming better understood. Kroger, et al. (Science, Volume 286, pages 1129–1132, Nov. 5, 1999; Proceedings of the National Academy of Science of the USA, Volume 97, Number 26, pages 14133–14138, Dec. 19, 2000) (incorporated herein by reference) have recently isolated polypeptides (called “silaffins”) and polyamines within the wall of a diatom (Cylindrotheca fusiformis). Silaffins have been found to be responsible for the precipitation of the silica nanoparticles within the frustule wall. Indeed, when these authors exposed a solution of silicic acid to a given silaffin, silica particles were rapidly formed by precipitation. Hence, silaffins are believed to act as biocatalysts for the precipitation of silica from seawater or fresh water environments. By varying the relative mixtures of silaffins and polyamines, along with solution pH, Kroger, et al. have been able to control the precipitation of silica from solutions of silicic acid, so as to produce spheres of varied diameter and with varied degrees of interconnectivity (i.e., ranging from loose, isolated microspheres or nanospheres to membranes of interconnected particles). These authors have also begun to identify the genomic sequences associated with these polypeptides and polyamines.
Recently, Brott, et al. (Nature, Volume 413, pages 291–293 (Sep. 20, 2001) (incorporated herein by reference) have generated silica/polymer composites with a well-controlled distribution of silica microspheres through the use of a patterned silaffin-derived biocatalyst. These authors prepared a solution containing a monomer and a 19-amino-acid peptide unit of a silaffin from the diatom Cylindrotheca fusiformis. They exposed this solution to a holographic laser pattern with alternating regions of high and low intensity. In the high intensity regions of the laser pattern, the monomer polymerized. Consequently, alternating rows of polymer and silaffin were produced with a pattern similar to that of the laser hologram. Subsequent exposure of this polymer/silaffin composite to a silicic acid solution resulted in the precipitation of silica particles and, hence, the formation of a polymer/silica composite with a pattern similar to that of the laser hologram. The well-controlled spacing of the silica particles in the layered silica/polymer composites allowed such composites to be used as optical diffraction gratings.
A significant current limitation in using diatom frustules for microdevices or microcomponents is the limited chemistry of the naturally-occurring frustules. Diatom frustules are comprised of amorphous, hydrated (opaline) silica. Although silica may be satisfactory for some microcomponents, silica has several unattractive characteristics, such as: 1) low toughness (i.e., silica being a brittle ceramic), 2) poor biocompatibility (e.g., silicosis, the tendency of fine silica to cause fibrosis of the lungs, may occur), 3) poor chemical compatibility upon exposure to basic oxides or basic oxide melts at high temperatures, 4) relatively low creep resistance at high temperatures (e.g., above about 1400° C. for pure, amorphous silica), 5) poor thermal cyclability (e.g., if amorphous silica is heated to a sufficient temperature and time so as to crystallize into quartz or cristobalite, then the resulting quartz or cristobalite will exhibit displacive transformations on heating and cooling with significant volume changes that are likely to result in cracking), and 6) poor resistance to erosion (e.g., from abrasive particles). Consequently, the porous amorphous silica in diatom frustules will not exhibit an appropriate combination of mechanical, thermal, biomedical, and chemical properties for a number of potential microdevice applications. Other examples of naturally-occurring microtemplates include the spicules of sponges (comprised of silica or calcium carbonate) and the shells of mollusks (comprised of calcium carbonate). Pure calcium carbonate also exhibits characteristics that are not attractive for a number of potential microcomponent applications, including: 1) low toughness, 2) poor high-temperature stability (i.e., CaCO3 decomposes to CaO(s) and CO2(g) at elevated temperatures), and 3) poor chemical compatibility upon exposure to acidic oxides or acidic oxide melts at high temperatures.
Accordingly, processing methods are needed that are capable of converting biologically-derived templates, such as silica microtemplates or calcium carbonate microtemplates, into microcomponents comprised of other materials with more appropriate and beneficial properties, while retaining the desired microtemplate shapes and fine (typically nanoscale) features.
Significant effort has been expended over the past several decades to develop low-cost methods for fabricating ceramic powders of varied composition that possess well-controlled size distributions. The rates of sintering and grain growth of ceramic powder preforms are strongly influenced by the size distribution of the ceramic powder. Hence, control over the powder size distribution is critical for fabricating ceramic bodies, or ceramic composite bodies, with controlled microstructures and shapes. Relatively little success has been achieved in developing methods for producing ceramic powders with controlled and complex shapes. New, low cost processing methods that are capable of producing ceramic powders with controlled and complex shapes, with controlled sizes, and with a variety of compositions are needed.